This invention relates to video display systems in general and in particular to the creation of images in pressure sensitive phase change material films. Still more particularly, this invention relates to a method and apparatus for generating multiple gray level video displays in pressure sensitive phase change material films.
Video display systems are well known in the prior art. The ubiquitous raster scanned cathode ray tube display is commonly utilized in television and other video displays and has been so utilized for many years. Recently attempts have been made to generate a "flat" display for televisions and the like by utilizing a very complex liquid crystal display system. Another recent development in the area of flat displays is the utilization of thermochromic or pressure sensitive films. These types of materials and others are known to exhibit optically discernible phase changes when subjected to selected temperatures, pressures or electrical fields.
An excellent example of this type of display system may be seen in U.S. Pat. No. 4,236,156, issued to the inventor herein. This patent discloses a technique for writing and erasing data into a thin film of thermochromic or pressure sensitive film utilizing a combination of lasers and/or surface acoustic waves to affect an optically discernible phase change in the film material.
Another example of this type of display system is disclosed in U.S. Pat. No. 4,009,488, issued to Ronald S. Smith. The Smith patent discloses a video display system which includes a stress wave propagating apparatus which defines a viewing plane and includes a device for initiating propagation of stress waves in a first scanning direction. A light emitting apparatus is utilized to provide a second scan line and the emitted light is modulated by a video signal and interacts with the stress wave to result in a video display.
Each of the aforementioned display systems operates in an acceptable manner; however, such systems all exhibit a common shortcoming. While it is well within the scope of such systems to "switch" a single image element or "pixel," the complexity and quantity of image elements necessary to provide multiple gray levels within an image is a very difficult technical problem to overcome. In order to provide multiple gray levels it is necessary to vary the intensity of an image element or pixel which is barely visible to the human eye. As might well be imagined, the sheer number of such elements necessary for a reasonable sized display is overwhelming.